1. Field of the Invention
The present invention relates to an analog quadrature modulator (AQM), and more particularly to an apparatus and a method for compensating an error for an AQM.
2. Background of the Related Art
A power amplifier is used to amplify a high frequency signal. A power amplifier that amplifies only a strength of a signal linearly without distorting the signal is an ideal power amplifier. However, power amplifiers include a plurality of active elements that have nonlinear characteristics. Therefore, the amplified signal can be distorted, and a function of an entire system is severely affected.
To improve the nonlinear characteristics of the power amplifier, various corrections have been suggested. These include a feed forward method, an envelope feedback method, a predistortion method, and a bias compensation method. The predistortion method is frequently used since it is inexpensive for its function and can operate in wider bandwidth.
The predistortion method is able to improve the linearity of the system by predistorting an input signal to have opposite characteristics of the nonlinear distortion characteristic of the power amplifier. Also, the predistortion method can be realized on a baseband. Consequently a size of the entire system can be miniaturized and the efficiency can be improved.
In order to realize a wider predistortion system, the entire system is implemented using AQM rather than using digital quadrature modulation (DQM). However, the AQM system is constructed using analog elements, and includes error components such as DC offset and amplitude/phase imbalance. These are the principal elements that degrade the function of predistorter. Therefore, the error of the AQM should be compensated in order to get optimal function of the predistorter.
FIG. 1 is a diagram showing a related art error compensating apparatus for the AQM. As shown in FIG. 1, a path from the predistorter 110 to a directional coupler 105 is a main path on which a signal is transmitted. A path from the directional coupler 105 to a controller 107 is a feedback path for detecting an error generated in the AQM. The error is generated in the AQM due to the DC offset or the amplitude/phase imbalance.
The related art error compensating apparatus for the AQM includes a predistorter 110 for distorting a signal to have an opposite characteristic of the nonlinear characteristic of a digital input signal, and an error compensating unit 120 for compensating I/Q digital signals outputted from the predistorter 110 by as much as an error compensating signal. The apparatus further includes first and second digital/analog converters 102 and 103 for converting the I/Q digital signals outputted from the error compensating unit 120 into I/Q analog signal respectively, and a modulator 130 for modulating frequencies of the analog signal outputted from the first and second D/A converters 102 and 103. Next, a power amplifier 104 is provided for amplifying the output signal of the modulator 130 and supplying the signal to the directional coupler 105. The apparatus further includes a down converter 140 for down converting a returning signal branched in the directional coupler 105, an analog/digital converter 106 for converting the output signal of the down converter 140 into a digital signal and a controller 107 for detecting an error compensating value by comparing the output signal (Vfb) of the analog/digital converter 106 to the I/Q digital signals (Vref) inputted from the predistorter 110, and applying the error compensating signal according to the value to the error compensating unit.
The error compensating unit 120 is an equivalent circuit of the modulator 130, and includes a first amplifier 121 for controlling gain of an I-digital signal, which is predistorted according to a first gain compensating signal (α) transmitted from the controller 107, and a second amplifier 122 for controlling gain of a Q-digital signal, which is predistorted according to a second gain compensating signal (β) transmitted from the controller 107. The compensating unit 120 further includes a third amplifier 123 for controlling a phase of an output signal from the second amplifier 122 according to a first phase compensating signal (sin φ), and a fourth amplifier 125 for controlling a phase of an output signal from the second amplifier 122 according to a second phase compensating signal (cos φ). Additionally included are a first adder 124 for adding output signals of the first amplifier 121 and the third amplifier 123, a second adder 126 for adding an output signal of the first adder 124 and a first DC offset signal (C1), and a third adder 127 for adding an output signal of the fourth amplifier 125 and a second DC offset signal (C2).
The modulator 130 inlcudes a first multiplier 131 for multiplying an I-analog signal outputted from the first D/A converter 102 by a local oscillator frequency outputted from a local oscillator, a second multiplier 132 for multiplying a Q-analog signal outputted from the second D/A converter 103 by the local oscillator signal outputted from the local oscillator and phase converted as 90°, and a combiner 133 for combining the output signals of the first and second multipliers 131 and 132 and outputting a high frequency signal.
The controller 107 calculates an AQM error using a reference signal (Vref) inputted from the predistorter 110 and a signal (Vfb) returned from the directional coupler 105, and compensates the respective errors.
An operation of the related art error compensating apparatus for the AQM will be described as follows.
First, the predistorter 110 controls a level of an inputted digital signal, and inputs the signal into the error compensating unit 120 after distorting the signal into I/Q digital signals having opposite characteristics of the nonlinear characteristics of the power amplifier.
The error compensating unit 120 compensates the error of the I/Q digital signals outputted from the predistorter 110 and applies the signal to the first and second D/A converters 102 and 103. The first and second D/A converters 102 and 103 then output the inputted I/Q digital signals after converting them into I/Q analog signals.
The modulator 130 receives the I/Q analog signals outputted from the first and second D/A converters 102 and 103 and AQM modulates the signals. That is, the first multiplier 131 of the modulator 130 up converts the I-analog signal outputted from the first D/A converter 102 by multiplying the signal by the local oscillator signal outputted from the local oscillator, and the second multiplier 132 up converts the Q-analog signal outputted from the second D/A converter 103 by multiplying the signal by a signal having phase difference of π/2 from the local oscillator signal. The respective signals which were up converted are combined as a high frequency signal in the combiner and applied to the power amplifier 104.
The down converter 140 down converts the frequency of a signal which is branched and returned from the directional coupler 105 after passing through the power amplifier 104, and applies the signal to the A/D converter 106. The A/D converter 106 converts the output signal of the down converter 140 into the digital signal outputs it into the controller 107.
The controller 107 detects the error value by performing a predetermined calculation with the I/Q digital signals (Vref) inputted from the predistorter 110 and the I/Q digital signals (Vfb) received from the A/D converter 106. The error compensating signal is then applied to the error compensating unit 120 for compensating the error value. The error compensating unit 120 compensates the error of the I/Q digital signals according to the error compensating signal and outputs the compensated signal.
A process of detecting the error compensating signal by the controller will be described in additional detail as follows.
The controller 107 detects respective DC offsets (C1 and C2) from the returned I/Q digital signals, removes the DC offset for the returned I/Q digital signals, and calculates gain compensating signals (α and β) by comparing the I/Q digital signals, from which the DC offset is removed, to the I/Q digital signals (Vref), which is a reference signal inputted from the predistorter 110. The gain is thereby compensated.
A time delay value is detected by using the gain compensated I-digital signal and the I-digital signal, which is a reference signal inputted through the predistorter 110. After that, the controller compensates the time delay of the gain compensated I/Q digital signals. In addition, the phase compensating value (φ) is detected by using the Q-digital signal, of which the time delay is compensated, and using the Q-digital signal, which is a reference signal inputted through the predistorter 110. The Q-digital signal, of which the time delay is compensated, is compensated by as much as the phase compensating value.
The error compensating unit 120, which received the error compensating signal detected through above processes, compensates the gain of the I/Q digital signals through the first and second amplifiers 121 and 122. It also compensates the phase error of the I/Q digital signals through the third and fourth amplifiers 123 and 125, and compensates the DC offset of the I/Q digital signals through the second and third adders 126 and 127. Finally it outputs the digital signal.
As described above, the related art AQM error compensating apparatus has various problems. For example, the AQM error compensating apparatus detects the error compensating signal by comparing the reference digital signal inputted from the predistorter to the returned digital signal. Therefore, memories for storing the respective digital signals are needed. In addition, the time delay must be precisely compensated before calculating the AQM error value. That is, the error in the time delay affects to the gain and phase error values. Therefore, the time delay should be precisely compensated. However, it is difficult to realize the precise time delay compensation.
In addition, in the related art AQM compensating apparatus, in order to detect a precise phase compensating value, many interpolations are required. However, it is difficult to get many interpolations due to the memory limit in the controller.
Also, in the related art AQM error compensating apparatus, the error compensating unit performs the gain compensation of the inputted digital signal earlier than the phase compensation. Therefore, independent functions between the gain and phase are not performed, and accuracy of the compensating function is reduced.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.